This invention is directed generally to the field of RF (Radio Frequency) communication, and more particularly to two-way RF communication between a base station and a subscriber unit such as a selective call transceiver.
The coverage area of a conventional RF communication system is typically divided into zones and sub-zones in which a subscriber unit may be located. For efficient communication with a particular subscriber unit, the system needs to know where that unit is located. This permits communication to be established between a base station that is located in the same zone or sub-zone that the subscriber unit is located in.
To locate a given subscriber unit in one type of two-way communication system, the system may simulcast a WRU (where are you?) signal over a forward channel, asking the addressed subscriber unit to respond on a reverse channel. If the subscriber unit responds properly, base station receivers can determine where the responding unit is located by, for example, detecting an error-free response at a location within the coverage area. This allows the system to send a message to the subscriber unit via a transmitter that is located in close proximity to the subscriber unit.
As a subscriber unit moves from one zone or sub-zone to another, the subscriber unit typically registers with the system by transmitting registration information back to the base station receivers that are located in the subscriber unit""s new zone or sub-zone. In a 2-way paging system, for example, a subscriber unit registers with the system via an ALOHA registration packet that the unit transmits on the reverse channel. By detecting the registration packet, the system knows where the subscriber unit is located and can address the subscriber unit in the zone or sub-zone where the unit is known to be located.
A problem arises when the subscriber unit experiences a loss of the forward channel signal. A subscriber unit may lose the forward channel signal when entering a tunnel, for example, and re-acquire the signal within the same zone or sub-zone after emerging from the tunnel. During the time of signal loss, the system may have tried (and failed) multiple times to locate the subscriber unit or to send a message to it. The system would then have an undelivered message intended for a subscriber unit of unknown location.
It is desired to solve the foregoing problem in a manner that avoids flooding the reverse channel with unnecessary attempts to re-register, but allows communication to be re-established with subscriber units that are truly lost.